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Mercury, the solar system’s smallest planet with a diameter of 4,879 km, is slowly shrinking. While Earth is a mid‑sized planet that could contain 20 Mercurys inside its volume, astronomers now have evidence that Mercury’s size is diminishing.
NASA’s Mariner 10, launched in November 1973, made the first fly‑by of Mercury in early 1974, capturing images of 45 % of its surface. Subsequent fly‑bys revealed surface wrinkles that hinted at contraction. In 2004, the Messenger probe approached Mercury three times between 2008 and 2009, then entered orbit in 2011, providing high‑resolution images of steep ridges and the planet’s complex topography.
By measuring the surface wrinkles and ridges observed by Mariner 10, scientists estimated a contraction of 2–3 km (1.2–1.9 mi) since the planet’s formation. Messenger’s improved imaging, however, revealed a much larger shrinkage, aligning with computer models that predict 10–20 km (6.2–12.4 mi). Current data place Mercury’s diameter reduction at roughly 11 km (≈7 mi).
Mercury’s unusually large core—mostly iron—retains a liquid portion, as radar observations from Earth have shown. The core is cooling and contracting, causing the planet’s outer layers to respond with ridges and wrinkles. This process mirrors the rapid heat transfer in a small body, where a cooling core directly affects the surface gravity and morphology. No other planet has shown comparable core‑driven shrinkage.
Formed about 4.5 billion years ago, Mercury has been steadily contracting. While early Mariner 10 data suggested minimal shrinkage, modern analysis from Messenger confirms a significant size loss consistent with theoretical models. Understanding Mercury’s contraction offers insights into planetary evolution and core dynamics across the solar system.
